The present invention relates to lateral shearing interferometers (LSI) for making highly accurate measurements of wavefront aberrations. The invention overcomes the inaccuracies associated with conventional implementations of grating-based LSIs.
The following publications are cited in this application as superscript numbers:
1. G. E. Sommargren, xe2x80x9cDiffraction methods raise interferometer accuracy,xe2x80x9d Laser Focus World, 32, 61-71, (8/96).
2. A. K. Ray-Chaudhuri, et al, xe2x80x9cAlignment of a multilayer-coated imaging system using extreme ultraviolet Foucault and Ronchi interferometric testing,xe2x80x9d J. Vac Sci Technol. B, 13, 3089-3093 (1995).
3. H. Medecki, et al, xe2x80x9cPhase-shifting point diffraction interferometer,xe2x80x9d Opt. Lett., 21, 1526-1528 (1996).
4. P. Naulleau et al, xe2x80x9cCharacterization of the accuracy of EUV phase-shifting point diffraction interferometry,xe2x80x9d in Emerging Lithographic Technologies II, Yuli Vladimirski, Editor, Proceedings of SPIE Vol. 3331, 114-123, (1998).
5. P. Carre, xe2x80x9cInstallation et utilisation du comparateur photoelectric et interferential du bureau international des poids et mesures,xe2x80x9d Metrologia, 2, 13-17 (1966).
6. R. Crane, xe2x80x9cInterference phase measurement,xe2x80x9d Appl. Opt., 8, 538-542 (1969).
7. J. H. Bruning, et al, xe2x80x9cDigital wavefront measuring interferometer for testing optical surfaces and lenses,xe2x80x9d Appl. Opt., 13, 2693-2703 (1974).
8. M. Takeda, et al, xe2x80x9cFourier-transform method of fringe-pattern analysis for computer-based topography and interferometry,xe2x80x9d J. Opt. Soc. Am., 72, 156-160 (1982).
9. E. Leith, et al, xe2x80x9cElectronic holography and speckle methods for imaging through tissue using femtosecond gated pulses,xe2x80x9d Appl. Opt., 30, 4204-4210 (1991).
10. K. A. Goldberg, et al, xe2x80x9cA 3-D numerical study of pinhole diffraction to predict the accuracy of EUV point diffraction interferometry,xe2x80x9d OSA Trends in Optics and Photonics Vol. 4, Extreme Ultraviolet Lithography, G. D. Kubiac and D. R. Kania, eds, (Optical Society of America, Washington, D.C. 1996), pp. 133-137.
11. D. A. Tichenor, et al, xe2x80x9cDevelopment and characterization of a 10xc3x97 Schwarzschild system for SXPL,xe2x80x9d in OSA Proceedings on Soft X-Ray Projection Lithography, Vol. 18, A. M. Hawryluk and R. H. Stulen, eds., (Optical Society of America, Washington, D.C., 1993), pp. 79-82.
12. P. de Groot, xe2x80x9cDerivation of algorithms for phase-shifting interferometry using the concept of a data-sampling window,xe2x80x9d Appl. Opt., 34, 4723-4730 (1995).
13. K. Freischlad and C. Koliopoulos, xe2x80x9cFourier description of digital phase-measuring interferometry,xe2x80x9d J. Opt. Soc. Am. A, 7, 542-551 (1990).
14. Y. Surrel, xe2x80x9cDesign algorithms for phase measurements by the use of phase stepping,xe2x80x9d Appl. Opt., 35, 51-60 (1996).
15. J. Tome and H. Stahl, xe2x80x9cPhase-measuring interferometry: applications and techniques,xe2x80x9d in Optical Testing and Metrology II, Proceedings of SPIE Vol. 954, 71-77 (1988).
16. K. Creath, xe2x80x9cComparison of phase-measuring algorithmsxe2x80x9d in Surface Characterization and Testing, Proceedings of SPIE Vol. 680, 19-28 (1986).
17. H. Stahl, xe2x80x9cReview of phase-measuring interferometry,xe2x80x9d in Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection, Proceedings of SPIE Vol. 1332, 71-77 (1990).
18. J. E. Bjorkholm, et al., xe2x80x9cPhase-measuring interferometry using extreme ultraviolet radiation,xe2x80x9d J. Vac. Sci. and Technol. B, 13, 2919-2922 (1995).
19. P. Naulleau, et al., xe2x80x9cThe EUV phase-shifting point diffraction interferometer: a sub-angstrom reference-wave accuracy wave front metrology tool,xe2x80x9d Appl. Opt., 38, 7252-7263 (1999).
20. K. A. Goldberg, xe2x80x9cExtreme Ultraviolet Interferometry,xe2x80x9d Ph.D. dissertation (University of California, Berkeley, 1997).
21. D. Attwood, et al., xe2x80x9cTunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions,xe2x80x9d IEEE J. Quantum Electron., 35, 709-720 (1999).
22. V. Ronchi, xe2x80x9cForty years of history of a grating interferometer,xe2x80x9d Appl. Opt., 3, 437-451 (1964).
23. A. Lohmann and O. Bryngdahl, xe2x80x9cA lateral wavefront shearing interferometer with variable shear,xe2x80x9d Appl. Opt., 6, 1934-1937 (1967).
24. S. Yokozeki and T. Suzuki, xe2x80x9cShearing interferometer using the grating as the beam splitter,xe2x80x9d Appl. Opt., 10, 1575-1580 (1971).
25. J. C. Wyant, xe2x80x9cDouble frequency grating lateral shear interferometer,xe2x80x9d Appl. Opt., 12, 2057-2060 (1973).
26. H. O. Bartlett and Yajun Li, xe2x80x9cLau interferometry with cross gratings,xe2x80x9d Optics Comm., 48, 1-6 (1983).
27. J. C. Wyant, xe2x80x9cUse of an ac heterodyne lateral shear interferometer with real-time wavefront correction systems,xe2x80x9d Appl. Opt., 14, 2622-2626 (1975).
28. J. Schwider, xe2x80x9cSingle sideband Ronchi test,xe2x80x9d Appl. Opt., 20, 2635-2642 (1981).
29. J. Braat and A. Janssen, xe2x80x9cImproved Ronchi test with extended source,xe2x80x9d J. Opt. Soc. Am. A, 16, 131-140 (1999).
30. P. Naulleau and K. A. Goldberg, xe2x80x9cDual-domain point diffraction interferometer,xe2x80x9d Appl. Opt, 38, 3523-3533 (1999).
31. D. Malacara, Optical Shop Testing, (John Wiley and Sons, New York, 1992), pp. 346-348.
32. D. W. Sweeney, et al., xe2x80x9cEUV optical design for a 100 nm CD m imaging system,xe2x80x9d in Emerging Lithographic Technologies II, Y. Vladimirsky, ed., Proc. SPIE Vol. 3331, 2-10 (1998).
33. M. P. Rimmer, xe2x80x9cMethod for evaluating lateral shearing interferograms,xe2x80x9d Appl. Opt., 13, 623-629 (1974).
All of the above publications are herein incorporated by reference in their entirety to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference in its entirety.
The recent development of extreme ultraviolet (EUV) optics for use in next-generation lithography systems has led to several advancements in EUV interferometry.2,3,18 With a demonstrated reference-wavefront accuracy of better than xcexEUV/350 (0.04 nm at xcexEUV=13.4 nm)19, the phase-shifling point diffraction interferometer (PS/PDI)3,19,20 is believed to be the most accurate EUV interferometer available. Although the PS/PDI has been proven to have high-accuracy, broad applicability of the PS/PDI is severely limited by its small dynamic range and the fact that it must be used with a highly coherent EUV source such as undulator radiation.
An alternative to the PS/PDI, with relaxed coherence requirements, is the lateral shearing interferometer (LSI).22-29 The Ronchi interferometer22 is perhaps the simplest realization of the LSI. Although not yet fully characterized for accuracy at EUV, this type of interferometer has previously been used for at-wavelength characterization of EUV optics.2, 18 More recently, a carrier-frequency (off-axis) implementation of the Ronchi interferometer has been used in the characterization of an EUV Schwarzschild objective. Direct comparison of this carrier-frequency LSI to the PS/PDI has demonstrated a rms measurement agreement of xcx9cxcexEUV/70. However, the development of next-generation EUV lithography systems requires interferometry with accuracy preferably far exceeding xcexEUV/100.
A problem with the conventional Ronchi interferometer is that it produces many interfering beams causing confusion in the data analysis and limiting the accuracy of the device. Another problem with this simple interferometer, limiting its accuracy, is that it is susceptible to noise added by high-frequency components in the test-optic wavefront.
Various methods have been described to overcome the limitation caused by multiple interfering beams. One particularly simple and elegant solution is the single-sideband LSI.28 In operation of the single-sideband LSI, an illumination beam is spatially filtered by a pinhole in the object plane, thus illuminating the test optic with a nearly spherical wave. A grating beamsplitter is placed in front of the image plane, where the illuminating beam is focused. Two of the diffracted orders propagate through a single large window in an image-plane mask with the remainder of the diffracted orders being blocked by the mask. Typically the two orders are chosen to be the zero order and either the +1 or the xe2x88x921 order. The two beams propagate to the detector where they overlap. While the zeroth-order beam propagates to the detector in the same manner as it would if the grating were not present, the diffracted order propagates with an angular shear, leading to a (typically) small lateral displacement at the detector. In this way, the test beam is compared with a sheared (laterally displaced) copy of itself.
The image-plane window serves several roles. One role is to guarantee that only two grating orders reach the detector, thus the Talbot effect of fringe w localization31 becomes irrelevant. Without these windows the position of the grating would be limited to a discrete set of defocus planes. Another effect of having only two interfering grating orders is that the recorded fringe pattern will be sinusoidal instead of square, facilitating the fringe analysis. Finally, confusion of multiple beam interference is avoided because only two beams are allowed to interfere at the detector. In this simple two-beam interference case, analysis of the resultant interferogram reveals an approximation to the gradient of the test wavefront, or the derivative in the direction of the shear. The original wavefront can be recovered by combining gradients from two (or more) directions using a variety of well-known techniques such as the Rimmer method.33 Although the single-sideband LSI solves the multiple-beam interference problem it does not address the issue of noise susceptibility due to high-frequency features in the test-optic wavefront.
An alternative solution to the multiple-beam interference problem is to use double-frequency gratings where only the first-diffracted orders of the two constituent gratings overlap.25 This method, however, is not well suited to EUV interferometry due to the difficulty in fabricating the dual-high-frequency gratings. Achieving fill order separation when testing an EUV optic (xcex=13.4 nm) with a moderate numerical aperture (NA) of 0.1 would require a grating pitch of 67 nm with accuracy to a small fraction of that pitch. Also it does not address the issue of noise susceptibility due to high-frequency features in the test-optic wavefront.
A third solution to the multiple-beam interference problem is ac heterodyning (phase shifting).27,18 In this case, the grating is translated laterally, orthogonal to the grating lines, producing temporal modulation of the intensity at each pixel at the detector. Temporal filtering is used to eliminate higher-order interference terms. Achieving high accuracy with this method when square-wave gratings are used, and hence square-wave temporal modulation is produced, requires a large number of samples to be recorded with very accurate grating translation. Because EUV systems are typically limited to using square-wave (binary) gratings due to fabrication issues, this method does not provide a time-efficient solution. Again this method does not address the issue of noise susceptibility due to high-frequency features in the test-optic wavefront.
The present invention relates to a defocused implementation of the LSI in which an image-plane filter is employed to allow both phase-shifting and Fourier wavefront recovery methods to be used. Furthermore, the two wavefront recovery methods can be combined in a dual-domain technique providing suppression of noise added by self-interference of high-frequency components in the test-optic wavefront.
In one embodiment, the invention is directed to a method of generating an interference pattern with a lateral shearing interferometer that includes the steps of:
(a) directing a source of radiation toward a test optic provided in a test-optic region of the lateral shearing interferometer whereby the test optic focuses a beam of radiation to an image plane located downstream from the test optic;
(b) dividing the beam of radiation into a first output beam and a second output beam directed at different angles with respect to one another such that the first output beam impinges at a first location on the image plane and the second output beam impinges at a second location, that is laterally separated from the first location, on the image plane, wherein the first and second locations on the image plane onto which the first and second output beams impinge define a beam-separation angle;
(c) phase shifting at least one of the first output beam and the second output beam;
(d) passing the first output beam through a first window on a mask that is positioned at the image plane of the test optic to produce a first wave and passing the second output beam through a second window on the mask to produce a second wave;
(e) recording a set of interference patterns (interferograms), with relative phase shifting between each element of the set;
(f) recovering a first shearing wavefront by processing the recorded interferograms in both temporal and spatial domains;
(g) repeating steps (b) through (f) at at least one different beam-separation angle to recover at least one other shearing wavefront; and
(h) combining the shearing wavefronts to recover a test-optic wavefront.
In another embodiment, the invention is directed to a lateral shearing interferometer system that defines an optical path, said system including:
(a) an optical system under test;
(b) a source of electromagnetic energy that directs a beam of radiation toward the optical system which focuses the beam of radiation;
(c) a beam divider in the optical path for dividing the beam of radiation from the optical system into a first beam and a second beam;
(d) a mask that is positioned in the image plane of the optical system under test wherein the first beam passes through a first window on the mask and the second beam through a second window on the mask, wherein the first beam and second beam are directed at different angles with respect to one another such that the first beam impinges at a first location on the image plane and the second beam impinges at a second location, laterally separated from the first location, on the image plane, wherein the first and second locations on the image plane onto which the first and second beams impinge define a beam-separation angle;
(e) a phase-shifting mechanism for adjusting the phase of at least one of the first beam or second beam;
(f) a detector located downstream from the mask for recording a set of interference patterns (interferograms), with relative phase shifts between each element of the set;
(g) means for recovering the shearing wavefront by process the recorded interferograms in both temporal and spatial domains; and
(h) means for combing two or more shearing wavefronts that are measured at different beam-separation angles to recover a test-optic wavefront.